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The National Highway Traffic Administration says fires broke out in two of the cars' batteries after the undercarriage hit road debris. The debris damaged the batteries and caused a thermal reaction and fires. No one was hurt in the fires.

The probe affects more than 13,000 cars sold in the U.S. The agency says wants to examine risks from the undercarriage getting hit.

Tesla's batteries are located beneath the passenger compartment and protected by a quarter-inch-thick metal shield.

Tesla says it asked the government to investigate even though its cars catch fire at a far lower rate than gas-powered vehicles.

The investigation could lead to a recall, but no decision has been made.